Flow in the Lungs - Biofluidmechanics: An Introduction to Fluid Mechanics Macrocirculation and Microcirculation - page 256

FIGURE 9.2 Schematic of the lower respiratory system,

which is composed of the larynx, the trachea, and the bron-
chi. The trachea and the bronchi are surrounded by cartilage
to maintain the rigid shape of this portion of the respiratory
system.

Larynx

Trachea

Primary bronchi

Secondary bronchi

respiratory pressure changes. The tracheal cartilage wraps around the circumference of
the trachea, and the cartilage ends are connected by a smooth muscle cell. The contraction
and dilation of this smooth muscle cell changes the diameter of the trachea, which regu-
lates the tracheal air flow by either increasing or decreasing the resistance to flow. The
exterior walls of each bronchi is also wrapped with cartilage; however, the cartilage cover-
age is not as regular as seen along the trachea. This cartilage also acts to stiffen the con-
ducting pathways and can partially regulate air flow through this portion of the lungs.
The walls of bronchioles do not contain cartilage but do contain a large number of smooth
muscle cells to regulate the diameter of the vessel. Bronchioles are similar to the arterioles
of the cardiovascular system because they are the major resistance vessels to airflow in the
lungs. These vessels are innervated by the autonomic nervous system, which can induce
bronchodilation or bronchoconstriction, to change the airflow pattern throughout the
lungs.
Each terminal bronchiole is connected to multiple alveolar sacs via small respiratory
bronchioles. The alveolar sacs are common chambers that lead to the individual alveolus
( Figure 9.3 ). Combined, there are approximately 300 million alveoli in the lungs. Each
alveolus is directly associated with blood capillaries, allowing for a large surface area for
gas exchange (approximately 70 square meters total). Alveolar walls are composed of thin
epithelial cells. Macrophages (termed dust cells) monitor the lung epithelial surface to
engulf any foreign particles that make it through the respiratory track and into the alveoli.
A second cell type, the pneumocyte type II cell, is inter-dispersed throughout the epithelial
surface. These cells produce and excrete a surfactant, which is a viscous fluid mainly com-
posed of phospholipids. This surfactant covers the entire alveoli surface in a thin layer of
fluid, which acts to reduce the surface tension of the alveoli. If the surfactant is not pres-
ent, the surface tension increases to a level that would tend to collapse the alveoli, prevent-
ing gas exchange in the lungs. Therefore, the surfactant reduces the surface tension of the
respiratory surface to provide an large open area for gas exchange to occur between the
lungs and the cardiovascular system.
As we have stated, gas exchange occurs across the alveoli epithelial surface. Gases must
cross the respiratory boundary, which is composed of three layers to enter the blood